Deb's Unoffical WDW Guide is correct. Selling a multi-day ticket that has already been used at least once is a misdemeanor in the state of Florida. The relevant section is reprinted below in its entirety:
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<b>Florida Statutes <br>
817.361. Resale of multiday or multievent ticket</b>
Whoever offers for sale, sells, or transfers in connection with a commercial transaction, with or without consideration, any nontransferable ticket or other nontransferable medium designed for admission to more than one amusement location or other facility offering entertainment to the general public, or for admission for more than 1 day thereto, after said ticket or other medium has been used at least once for admission, is guilty of a misdemeanor of the second degree, punishable as provided in s. 775.082 or s. 775.083. A nontransferable ticket or other nontransferable medium is one on which is clearly printed the phrase: "Nontransferable; must be used by the same person on all days" or words of similar import. Upon conviction for a second or subsequent violation of this subsection, such person is guilty of a misdemeanor of the first degree, punishable as provided in s. 775.082 or s. 775.083.
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Now that makes it clear you can't re-sell a used multi-day pass. Can you give one away? That depends. If it's in a "commercial transaction", then it's irrelevant that you gave it away for free. Is giving a used pass to mom or dad a "commercial transaction"? Probably not. This particular statute has been around since 1988, but the term "commercial transaction" has never been interpreted by a court. Still, the language of the statute seems to clearly target those who deal in such used tickets for profitable reasons. So, in my opinion, it's legal to give your tickets away, so long as there's no hint of profit or commerce - in other words, it's truly a gift, not motivated for any business purpose [FYI, since I'm not licensed in the state of Florida, don't take this as legal advice please!].
Npw, my own personal thought on the morality of such a practice is that Disney is getting exactly what it bargained for: $200 today (or whatever the current price is) in exchange the right for one guest to be admitted to its parks on four days at any time in the future. Substituting my mom for me, or vice versa, is an economically uninteresting event. Nothing has changed on either side of the equation.
If this is truly a trivial economic event, then why is Disney so concerned about the transferabillity of its passes? Well, when someone buys a four-day park hopper and only uses two days, then Disney has a windfall. Disney took in your $200, but only needed to sink $100 into the park to keep it running. However, this is really a phantom windfall, for several reasons. First, those unused days never expire! It's only if I plan to visit WDW two days out of my whole life, and leave two days unused, that Disney sees any bonus. Since the vast, vast majority of park visitors will eventually use up all of their days within some commercially reasonable timeframe (say 2 years), this is of little or no benefit to Disney. Second, Disney doesn't know at the time of purchase that I'm only going to use two days, and waste the other two. Disney still has to keep its parks up and running as if every ticket holder was going to use their admission media to the fullest. Third, this simply isn't how Disney manages its parks. Disney doesn't count up the number of passes it sells, figure out how many guest-days it needs to provide service for, and then spend its money accordingly. The parks are always open, and Disney never knows how many people are going to show up. It's impossible to break down the cost of operating a park to the per capita/per day level. For these reason, Disney really can't use this windfall (or for that matter, even know of its existence).
What's the real reason then? I'll bet it's the corporate mindset. Disney saw people turning a profit selling Disney stuff, and they weren't getting a cut. Legitimate ticket resellers (by that I mean those who sell used passes and truthfully represent how many days are left on those passes) were making money on Disney's investment in the parks.
Now one could argue that Disney needs to control the re-selling because some people were getting ripped off, buying what they were told was a four-day hopper with two days left on it, only to find out at the turnstiles that it had zero days left. This would tend to make people upset with Disney for allowing such a thing to happen (along with the unscrupulous re-seller). But guess what, there's already a law out there that would criminalize such fraudulent activity. Disney didn't need this new law to allow Florida police to arrest sleazy re-sellers.
Given that Disney suffers no economic loss when you give your pass to a family member or friend, I wouldn't have a problem with it. Of course, just because I don't have a problem with it, doesn't mean others won't. And of course, I'd probably be leery to try such a thing out, knowing that the pass could be confiscated if Disney ever found out that I wasn't the original purchaser.
Whew! I think I have too much time on my hands.